1. Field of the Invention
The present invention relates to a reading method for an optical memory, and more particularly to a semiconductor laser driving method for suppressing a laser noise when optical information is read out.
2. Description of the Related Art
An optical memory which is an information recording medium such as a compact disk (CD) or a magneto-optical disk has a large recording capacity and a capability of high-speed access. Due to the above advantages, the optical memory is currently used as a recording device for various information equipments such as a personal computer or a word processor.
The information recorded on the information recording face of the optical memory is read out by using an optical unit which is generally referred to as an optical pickup. The information is read out by the optical pickup in the following manner. Laser light is emitted from a semiconductor laser which serves as a light source. The laser light is reflected by the information recording face while the intensity of the reflected laser light is modulated depending on the information recorded on the information recording face of the optical memory. The intensity of the reflected light is detected by a photoreceiving device which is provided in the optical pickup.
In the optical pickup having such a construction, the semiconductor laser is located at the side of the photoreceiving device. Accordingly, part of the laser light reflected from the information recording face of the optical memory returns to the semiconductor laser. Such part of laser light which returns to the laser resonator of the semiconductor laser is generally referred to as "feedback light". The feedback light may cause an optical intensity noise in the semiconductor laser.
The noise caused by the feedback light is generated due to the lightwave interference effect (hereinafter referred to as the interference effect) of the oscillating laser light and the feedback light in the inside of the semiconductor laser. More specifically, the feedback light disturbs the phase of the oscillating laser light in the inside of the semiconductor laser resonator at random, so that the optical output intensity of the laser light is slightly changed. Thus, an optical intensity noise is generated. That is, the laser light emitted from the semiconductor laser maintains the coherent condition (the coherent length of the semiconductor laser is 10-50 m), while the laser light travels to be reflected from the information recording face of the optical memory and returns to the semiconductor laser (the traveling distance is 30-100 mm). Thus, there occurs the interference of the oscillating laser light and the feedback light, so that a noise is generated. In general, the level of the noise increases, as the coherence of the laser light increases.
The noise caused by the feedback light (hereinafter referred to as the feedback light noise) deteriorates the carrier to noise ratio (the C/N ratio) of the system when the information recorded on the information recording face of the optical memory is to be read out. Therefore, the feedback light noise is a critical problem for the various systems such as a high-density magneto-optical disk system in which the intensity of signal light is low, an analog-recording laser disk system on which image information requiring a high S/N ratio is recorded, an optical memory reading apparatus which applies a micro optical pickup in which the amount of the feedback light to the semiconductor laser is large.
As one means for solving the above problem, the use of an optical isolator is considered. However, in the optical memory system field, there exist strong desires that the system price be suppressed to be low, and that the optical pickup is made smaller in size and weight. With these desires, it is not preferable to additionally provide an optical device such as an optical isolator, because such provision increases the price, the size and the weight of the resulting system.
As mentioned above, the feedback light noise is caused by the coherence of the semiconductor laser. Accordingly, in order to suppress the feedback light noise, it is effective to shorten the coherent length of the semiconductor laser. For such a purpose, an optical pickup of an optical memory device conventionally adopts a technique referred to as a high frequency superimposition method for modulating the intensity of the output laser light from the semiconductor laser at a high frequency, or a technique referred to as a self-oscillation laser method for naturally providing, in addition, a function of modulating the intensity of the output laser light.
FIG. 7 shows a semiconductor laser driving circuit which adopts the high frequency superimposition method. The semiconductor laser driving circuit includes an automatic power control circuit 70, a high frequency oscillation circuit 71, and a semiconductor laser device 72. According to a general method for driving a semiconductor laser device, the automatic power control circuit is directly coupled with the semiconductor laser device. Thus, the semiconductor laser device is driven by a DC current supplied from the automatic power control circuit. According to the high frequency superimposition method, the semiconductor laser device 72 is driven by a current which is obtained by superimposing a high frequency current from the high frequency oscillation circuit 71 on the DC current from the automatic power control circuit 70.
In the case where the semiconductor laser device 72 is driven by the current including such high frequency components, the frequency of the output laser light is modulated due to the non-linear effect inherent in the semiconductor laser device 72. Note that the frequency of light is greatly higher than the frequency of a usual electric signal. However, it is possible to treat, as the electric signal, a light signal spectrum the frequency of which is modulated by a signal having a certain frequency component, and side bands are formed on both the sides of the carrier. Accordingly, the signal spectrum is widened as compared with the spectrum of the output laser light from the semiconductor laser device driven by the DC current. This means that the coherence of the output laser light of the semiconductor laser device 72 is reduced.
It is preferable to set the frequency of the high frequency current which is superimposed on the DC current for driving the semiconductor laser device 72 so as to have a value sufficiently larger than the frequency band (1-5 MHz) of the reproduction signal of the recorded information. In general, the frequency is selected to have a value in the range of 50 to 500 MHz. As a result, the widths of the respective longitudinal mode spectra of the semiconductor laser device 72 which is driven by the current in which the signal having such a frequency is superimposed on the DC current are 10 GHz or higher. The coherent length of the laser light in this case is in the range of 1 mm to about 10 mm, which is remarkably lower than the value (10-50 m) in the case of the general DC current driving method. In the semiconductor laser device 72 having the coherent length in the range of 1 mm to about 10 mm, even if the feedback light which returns to the semiconductor laser device 72 with a traveling distance of 30-100 mm reaches the inside of the laser resonator, the feedback light neither interferes with the oscillating laser light in the laser resonator, nor causes a feedback light noise.
On the other hand, according to the self-oscillation laser method, one semiconductor laser has, in addition, the same effect as that attained by the above high frequency superimposition method. In this case, the high frequency oscillation 71 is realized in the inside of the laser resonator as the fluctuation of the carrier in the semiconductor laser device 72. Therefore, it is not necessary to additionally provide a high frequency oscillation circuit.
On the other hand, by the high frequency superimposition method, it is necessary to provide the high frequency oscillation circuit 71. Thus, this case has a drawback that the driving circuit for the semiconductor laser device 72 is expensive.
The self-oscillation laser method has an advantage that it is unnecessary to additionally provide a high frequency oscillation circuit. However, the acceptable design conditions for a semiconductor laser device having the effect of the high frequency oscillation by the fluctuation of the carrier are seriously limited. Therefore, the semiconductor laser device is likely to be adversely affected by the variation in the process conditions during the fabrication of the semiconductor laser device, and thus the actually fabricated semiconductor laser device may have values which are slightly varied from the design values. This leads to a drawback that the yield thereof decreases.
Further, the high frequency superimposition method and the self-oscillation laser device have the same problem as follows. In both the methods, the frequency of the output laser light from the semiconductor laser device is modulated by the current including the high frequency components. In these case, the axial mode (spectrum) of the output laser light is generally a multi mode as shown in FIG. 8. In other words, the spectrum has a plurality of peaks, so that the total spectral width is widened to be about 3 nm in wavelength (i.e., 1.5 THz in optical frequency).
Therefore, in addition to the deterioration in the coherent length, the monochromaticity of the output laser light is deteriorated more than necessary. Such deterioration in the monochromaticity of the laser light constitutes a problem when the semiconductor laser device is applied to an optical pickup including a condensing or polarizing optical system which utilizes a diffraction phenomenon of light such as a hologram, a grating, or the like which is recently used in this field. This is because the light control function by the hologram or the grating largely depends on the employed laser light wavelength, and because the monochromaticity of 1 nm or less is generally required.